Conventionally, as a cryogenic refrigerating machine of such kind, there is well known a GM (Gifford-McMahon) refrigerating machine in which a displacer for forming an expansion space in a cylinder is provided and the reciprocation of the displacer expands high-pressure working gas supplied to the expansion space to produce a cold condition at an extremely low temperature level and discharges expanded low-pressure working gas from the expansion space to the outside of the cylinder.
Further, for example, Japanese Patent Application Laid-Open Gazette No. 6-300378 discloses a machine-driven type GM refrigerating machine in which a displacer is connected to a motor through a crank shaft and is reciprocated by the activation of the motor. In this refrigerating machine, there is proposed a technique that a valve body, which makes sliding contact with a valve plate rotating together with the crank shaft as a single piece and makes the valve plate open and closed, is configured to be rotatable from the outside and that by changing a position of the valve body relative to the valve plate, timing for supplying high-pressure working gas to the expansion space of the cylinder and timing for discharging low-pressure working gas expanded in the expansion space are variable in association with each other.
Furthermore, there is also known a gas pressure-driven type (improved Solvay type) GM refrigerating machine in which an intermediate-pressure room set at an intermediate pressure between high and low pressures is formed in a cylinder and a piston is reciprocated together with a displacer by a difference in gas pressures between the intermediate-pressure room and an expansion space.
Since the above-described gas pressure-driven type GM refrigerating machine drives the displacer by the gas pressure difference, in order to achieve a smooth operation of the displacer, the refrigerating machine is generally configured such that a high-pressure valve-open position for supplying high-pressure working gas to the expansion space in the cylinder and a low-pressure valve-open position for discharging the working gas in the expansion space substantially have the same time ratio (approximately 50%) in one reciprocation cycle of the displacer.
However, from a study of the capacity of the above-described refrigerating machine, the inventors have found, in terms of increase in the refrigerating capacity, that it is not necessarily required that the time ratio is substantially equalized between the high-pressure valve-open position and the low-pressure valve-open position and that it prevents increase in the refrigerating capacity instead.
Accordingly, an object of the present invention is, in a cryogenic refrigerating machine for producing a cold condition at an extremely low temperature level by the reciprocation of a displacer as described above, to provide an increased capacity of the cryogenic refrigerating machine by appropriately changing a time ratio of a low-pressure valve-open position for discharging working gas in an expansion space of a cylinder.